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<title>Consequences of using poly_int (GNU Compiler Collection (GCC) Internals)</title>

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<a name="Consequences-of-using-poly_005fint"></a>
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<hr>
<a name="Consequences-of-using-poly_005fint-1"></a>
<h3 class="section">10.2 Consequences of using <code>poly_int</code></h3>

<p>The two main consequences of using polynomial sizes and offsets are that:
</p>
<ul>
<li> there is no total ordering between the values at compile time, and

</li><li> some operations might yield results that cannot be expressed as a
<code>poly_int</code>.
</li></ul>

<p>For example, if <var>x</var> is a runtime invariant, we cannot tell at
compile time whether:
</p>
<div class="smallexample">
<pre class="smallexample">3 + 4<var>x</var> &lt;= 1 + 5<var>x</var>
</pre></div>

<p>since the condition is false when <var>x</var> &lt;= 1 and true when <var>x</var> &gt;= 2.
</p>
<p>Similarly, <code>poly_int</code> cannot represent the result of:
</p>
<div class="smallexample">
<pre class="smallexample">(3 + 4<var>x</var>) * (1 + 5<var>x</var>)
</pre></div>

<p>since it cannot (and in practice does not need to) store powers greater
than one.  It also cannot represent the result of:
</p>
<div class="smallexample">
<pre class="smallexample">(3 + 4<var>x</var>) / (1 + 5<var>x</var>)
</pre></div>

<p>The following sections describe how we deal with these restrictions.
</p>
<a name="index-poly_005fint_002c-use-in-target_002dindependent-code"></a>
<p>As described earlier, a <code>poly_int&lt;1, <var>T</var>&gt;</code> has no indeterminates
and so degenerates to a compile-time constant of type <var>T</var>.  It would
be possible in that case to do all normal arithmetic on the <var>T</var>,
and to compare the <var>T</var> using the normal C++ operators.  We deliberately
prevent target-independent code from doing this, since the compiler needs
to support other <code>poly_int&lt;<var>n</var>, <var>T</var>&gt;</code> as well, regardless of
the current target&rsquo;s <code>NUM_POLY_INT_COEFFS</code>.
</p>
<a name="index-poly_005fint_002c-use-in-target_002dspecific-code"></a>
<p>However, it would be very artificial to force target-specific code
to follow these restrictions if the target has no runtime indeterminates.
There is therefore an implicit conversion from <code>poly_int&lt;1, <var>T</var>&gt;</code>
to <var>T</var> when compiling target-specific translation units.
</p>


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